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Background  Plasmids play a crucial role in the evolution of bacterial genomes by mediating horizontal gene transfer. However, the origin and evolution of most plasmids remains unclear, especially for megaplasmids. Strains of the  Bacillus cereus  group contain up to 13 plasmids with genome sizes ranging from 2 kb to 600 kb, and thus can be used to study plasmid dynamics and evolution.    Results  This work studied the origin and evolution of 31  B. cereus  group megaplasmids (>100 kb) focusing on the most conserved regions on plasmids, minireplicons. Sixty-five putative minireplicons were identified and classified to six types on the basis of proteins that are essential for replication. Twenty-nine of the 31 megaplasmids contained two or more minireplicons. Phylogenetic analysis of the protein sequences showed that different minireplicons on the same megaplasmid have different evolutionary histories. Therefore, we speculated that these megaplasmids are the results of fusion of smaller plasmids. All plasmids of a bacterial strain must be compatible. In megaplasmids of the  B. cereus  group, individual minireplicons of different megaplasmids in the same strain belong to different types or subtypes. Thus, the subtypes of each minireplicon they contain may determine the incompatibilities of megaplasmids. A broader analysis of all 1285 bacterial plasmids with putative known minireplicons whose complete genome sequences were available from GenBank revealed that 34% (443 plasmids) of the plasmids have two or more minireplicons. This indicates that plasmid fusion events are general among bacterial plasmids.    Conclusions  Megaplasmids of  B. cereus  group are fusion of smaller plasmids, and the fusion of plasmids likely occurs frequently in the  B. cereus  group and in other bacterial taxa. Plasmid fusion may be one of the major mechanisms for formation of novel megaplasmids in the evolution of bacteria.

Evolution and dynamics of megaplasmids with genome sizes larger than 100 kb in the Bacillus cereus group